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Math Help - Help with solving linear systems by substitution

  1. #1
    Junior Member risteek's Avatar
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    Help with solving linear systems by substitution

    Okay I was doing well until I stumbled across a question that was backwards if that makes sense.

    Like:
    a+4b=15
    5b=2a+3

    or

    5=2y-x
    7=3y-2x

    I have been trying at those two for over an hour.

    Please help!
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  2. #2
    Junior Member risteek's Avatar
    Joined
    Mar 2008
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    Canada, with the penguins
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    Listen I have been this for 5 hours straight I am a poor math student but I try. My teacher is rude and ignorant. Please if it is not to much trouble a little help please. Currently I have no patience.
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  3. #3
    Member
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    Quote Originally Posted by risteek View Post
    Okay I was doing well until I stumbled across a question that was backwards if that makes sense.

    Like:
    a+4b=15
    5b=2a+3

    or

    5=2y-x
    7=3y-2x

    I have been trying at those two for over an hour.

    Please help!
    I'll do the first one.

    (1) a+4b=15
    (2) 5b=2a+3

    Isolate for either "a" or "b" in (1) or (2).
    I'll isolate for "a" because it's easier.

    (1) a = 15 - 4b
    (2) 5b = 2a + 3

    Now, substitute (1) into (2) to find "b"

    5b = 2a + 3
    5b = 2(15 - 4b) + 3
    5b = 30 - 8b + 3
    Isolate for "b"
    5b + 8b = 33
    13b = 33
    b = 33/13

    Now, substitute "b = 33/13" into (1) or (2) to find "a"

    a = 15 - 4b
    a = 15 - 4(33/13)
    a = 15 - 132/13
    Need a common denominator
    a = (195 - 132)/13
    a = 63/13
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